Listing of Poster Abstracts

Luminosity Distance for Born-Infeld Electromagnetic Waves Propagating in a Cosmological Magnetic Background

Gabriel R. Bengochea (IAFE-UBA-CONICET)

The discovery of a diminution in the observed energy fluxes coming from supernovae type Ia has opened one of the most puzzling and deepest problems in cosmology today. Although the cosmological constant seems to be the simplest explanation for the phenomenon, several dynamical scenarios have been tried out. We have studied a speculative model by analyzing Born-Infeld electromagnetic waves interacting with a static magnetic background in an expanding universe. Here we show how the non-linear character of Born-Infeld electrodynamics modifies the relation between the energy flux and the distance to the source, which gains a new dependence on the redshift that is governed by the background field. We compute the luminosity distance as a function of the redshift and compare with the Maxwellian curves for supernovae type Ia.

Probing Dark Energy Inhomogeneities with Supernovae

Michael Blomqvist (Stockholm University)

The search for spatial variations in dark energy properties is an important complement to studies aiming at constraining the time evolution of dark energy. We discuss the possibility to identify anisotropic and/or inhomogeneous cosmological models using type Ia supernova data. A search for correlations in current type Ia peak magnitudes over a large range of angular scales yields a null result. However, the same analysis limited to supernovae at low redshift, shows a feeble anticorrelation at angular scales of about 40 degrees. Upcoming data from, e.g. the SNLS and the SDSS-II supernova searches will improve our limits on the size of - or possibly detect - possible correlations also at high redshift at the percent level in the near future. With data from the proposed SNAP satellite, we will be able to detect the induced correlations from gravitational lensing on type Ia peak magnitudes on scales less than a degree.

Constraints on Modified Gravity Theories from Existing Geometrical and Large Scale Structure Growth Data

Ms.  Wenjuan Fang (Physics Department, Columbia University)

The observed acceleration of the cosmic expansion rate could be produced by modifications to general relativity on large spatial scales, rather than by the presence of dark energy. We compute CMB anisotropies in the so--called DGP model for modified gravity, by modifying the public Boltzmann code CAMB to include a parameterized post-Friedmann (PPF) description of the DGP model. We apply the Markov Chain Monte Carlo methods to fit the DGP model (both with and without curvature) to CMB temperature and polarization power spectra, as well as to luminosity distances to Type Ia Supernovae (SN), and Hubble Space Telescope measurements of $H_0$. We find that DGP models give a significantly poorer fit to the combined SNLS+HST+WMAP5 data than LCDM models with the same number of free parameters. The fits favor the LCDM model by a relative likelihood of $-2\Delta \ln L=28$ (if flat models are assumed with seven free parameter), and by $-2\Delta \ln L=21$ (when curvature is an 8th free parameter). We also find that in the flat models, $\approx$1/3rd of the relative likelihood is contributed by information on growth from the ISW effect, whereas growth accounts for $\approx$2/3rd in the curved case.

Model Independent Constraints on Mass-varying Neutrino Scenarios

Urbano Franca (IFIC - CSIC/Universidad de Valencia)

Models of dark energy in which neutrinos interact with the scalar field responsible for the acceleration of the universe in general imply the variation of the neutrino masses on cosmological time scales. In this work we propose a parameterization for the neutrino mass variation that captures the essentials of those scenarios and allows for constraining them in a model independent way, that is, without resorting to any particular scalar field model. Models in which the neutrino masses were heavier in the past are strongly disfavored, and the results are totally consistent with constant mass neutrinos.

Statistics of Dark Matter Halos in the Deep Lens Survey

Dr.  Jeffrey Kubo (Fermi National Accelerator Laboratory)

One proposed method with which future optical imaging surveys will constrain cosmological parameters, including dark energy, is with a survey of shear-selected galaxy clusters. In this method, peaks in the matter distribution are identified as a function of their signal-to-noise from weak lensing `mass' maps. Current deep optical surveys, such as the Deep Lens Survey are producing the first such maps from weak lensing. Here we present a weak lensing map of the Deep Lens Survey F2 field covering an area of four square degrees. To reconstruct the projected mass in this field we use a maximum likelihood approach which overcomes issues associated with traditional 'direct' methods of mass reconstruction. We present properties of high signal-to-noise dark matter peaks detected in the F2 field, as well as the global distribution of these peaks.

The Probability Distribution Function of Dark Matter in Redshift Space and a Reconstruction Method for the Initial Distribution Function

Mr.  Tsz Yan Lam (University of Pennsylvania)

The ellipsoidal collapse model is a modification of the Zeldovich Approximation which specifies how an initial fluctuation field can be mapped to a final one, even when the mapping is rather nonlinear. I show how this allows an accurate prediction of the real and redshift space smoothed dark matter probability distribution functions. This mapping also motivates a method for reconstructing the initial field from the nonlinear one. Application of this method to the highly non-Gaussian non-linear density field in a numerical simulation yields an accurate estimate of the initial Gaussian field from which it evolved.

Constraints on Dark Energy from the Growth of Structure Observed in X-ray Luminous Galaxy Clusters

Adam Mantz (KIPAC)

The growth of cosmic structure is a sensitive probe of cosmology, including dark energy or modified gravity. X-ray emission from the baryonic content of massive clusters of galaxies provides a means to measure the growth of structure in these large systems. Thanks to the ROSAT All Sky Survey, hundreds of massive, X-ray luminous clusters out to redshift z~1 have been catalogued. Using this data, we have obtained constraints on the dark matter density, amplitude of density perturbations, and dark energy equation of state which are consistent with the concordance model of cosmology. This method is complementary to other cosmological observations; in combination with cosmic microwave background, supernova and cluster gas fraction data, the constraint on the dark energy equation of state approaches the 5% level.

Calibrating Photometric Redshifts for Dark Energy Experiments with Cross-Correlation Techniques

Dr.  Jeffrey Newman (U. Pittsburgh)

To place constraints on the dark energy equation of state, many proposed experiments will rely critically on redshift distributions determined from photometric redshifts. The requirements are very stringent - redshift calibration uncertainties must be below 0.002(1+z) for Stage IV experiments. It is unlikely that it is possible to obtain such a robust calibration using conventional methods, as all deep spectroscopic samples are substantially (typically 10%-50% or more) incomplete, in a manner that depends on galaxy luminosity and color, even at magnitudes much shallower than future experiments will probe. In this talk, I present a new method which can meet the calibration requirements for future experiments with existing and currently-planned spectroscopic datasets; incomplete spectroscopy of the brightest objects at a given z is sufficient for this technique. Specifically, the clustering between galaxies in a photometric redshift bin and a spectroscopic sample, measured as a function of spectroscopic z, allows us to determine the true redshift distribution of objects in the bin to high accuracy, meeting or exceeding the calibration requirements of Stage IV experiments with realistic redshift samples.

Accurate Extragalactic Distances and Dark Energy

Dr.  Rob Olling (University of Maryland)

I discuss how the current and future uncertainty on H_0 affects the uncertainty in the equation of state (EOS) of dark energy (DE) (w). The same procedure yields an estimate for the attainable accuracy on the total- and curvature densities of the Universe. Also, a direct determination of H_0 provides a direct measure of the critical density. I present two limiting cases. In the more unrealistic case, the constraints provided by "additional" data do not improve significantly, while the error on H_0 is decreased by factors 2 -- 10. The other case, with improved additional data but with current H_0 errors has been investigated by the Dark Energy Task Force (DETF). In the former scenario, a PLANCK-like CMB determination hardly changes the accuracy with which w and Omega_tot are determined, unless H_0 is determined to a few percent. The DETF concluded that H_0 accuracy hardly matters if the additional data are sufficiently accurate. However, moderate H_0 improvements combined with moderately improved "other" data might significantly constrain the evolution of DE, but at a reduced cost and/or at a quicker pace. Also, this would avoid the very unattractive (and unnecessary, really) H_0-marginalization procedure. Several methods can yield extragalactic distances with errors of the order of several per cent: I discuss the current and future strengths and weaknesses of these methods. Specifically I review methods based on: 1) the velocity field, 2) maser, 3) light echos, 4) binary stars, and 5) the "rotational parallax" (RP). These methods rely on geometry rather than astrophysics or cosmology, so that their results are robust. Time permitting, I will describe the RP technique which can provide accurate (1%), single-step and bias-free distances to Local Group galaxies. These distances set the zero-points for other distance indicators which in turn would yield H_0. Achieving an accuracy of a few per cent for M31, M33 and the LMC requires +/- 10 km/s radial velocities and proper motions from future astrometric missions such as SIM and possibly GAIA.

Estimating Dust Extinction in Distant Galaxies Using Quasars

Linda Ostman (Stockholm University)

We report on how observations of quasars shining through foreground galaxies offer a way to probe the dust extinction curves of distant galaxies. We compare the observed colours of quasars reported in the literature with spectral templates reddened by different extinction laws and dust properties. We measure a difference in rest-frame B-V between the quasar images we study, and quasars without resolved foreground galaxies. This difference in colour is indicative of significant dust extinction in the intervening galaxy. Good fits to standard extinction laws were found for 22 images, corresponding to 13 different galaxies. Our fits imply a wide range of possible values for the total-to-selective extinction ratio, Rv. The distribution was found to be broad with a weighted mode of 2.4 and a FWHM of 2.7. Thus the bulk of the galaxies for which good reddening fits could be derived, have dust properties compatible with the Milky Way value (Rv=3.1).

Probing Dark Energy Using Future X-ray Galaxy Cluster Data

Dr.  David Rapetti (KIPAC (SLAC/Stanford))

Forthcoming large X-ray or SZ galaxy cluster surveys, from missions such as eROSITA-SpectrumRG or SPT/ACT/Planck, respectively, will find hot, X-ray luminous clusters out to high redshifts. Short snapshot observations with a new X-ray observatory with capabilities similar to those planned for the Constellation-X mission should then be able to identify a sample of ~500 hot (kT>~5keV), suitably relaxed systems; and later re-observed them with longer exposures (~20ks per cluster on average) to measure the X-ray gas mass fraction, fgas, to a precision of ~5 per cent. We examine the ability to constrain dark energy using such an fgas sample, and find that the fgas experiment offers a competitive and complementary approach to the best other large, planned dark energy experiments with a comparable Dark Energy Task Force figure of merit. If time permits, I will also present preliminary, new constraints on gravity at cosmological scales. For this analysis we use a convenient parameterization of departures from the current gravity theory, General Relativity, and present-day measurements of the growth of cosmic structure in X-ray luminous galaxy clusters from the MACS, BCS and REFLEX X-ray cluster samples.

Studying the Nature of Dark Energy with Galaxy Clusters

Dr.  Thomas Reiprich (Argelander Institute for Astronomy)

I'll present the latest results from our Chandra and XMM-Newton (and Suzaku) follow-up of a complete sample of the 60 X-ray brightest clusters in the sky (HIFLUGCS). Furthermore, I'll report on the status of the weak lensing follow-up of the luminous and high-z subsample of 40 clusters from the 400 square degree survey. The combination of both samples will be used to constrain the nature of dark energy through the evolution of the cluster mass function and merger frequency. Moreover, these high quality observations will be vital to constrain the observable--mass relations required for near future large X-ray cluster surveys like eROSITA.

Impact of Massive Neutrinos on Nonlinear Clustering and Degeneracy with Dark Energy Parameter

Dr.  Shun Saito (University of Tokyo)

Future galaxy redshift surveys will open up an exciting opportunity for precision determinations of neutrino masses as well as dark energy parameter, w. Neutrino mass and w can be measured by probing the neutrino free-streaming scale and the baryon acoustic oscillation (BAO) scale, respectively. It is discussed that expected neutrino mass and w are degenerate because neutrino free-streming scale is comparable to BAO scale, where nonlinear clustreing cannot be neglected. In the literaturte, however, recent progress of nonlinear theory does not include the effect of neutrino free-streaming. In this talk, we present the new approach to include properly the non-linear gravitational evolution of matter power spectrum for a mixed dark matter model (neutrinos plus cold dark matter). Based on the perturbation theory, we show that the suppression of the power spectrum amplitude caused by massive neutrinos is enhanced in the weakly nonlinear regime where standard linear theory ceases to be accurate. Due to this enhanced effect and the gain in the applicable range of the model prediction, the nonlinear model may enable a precision of sigma(m_tot)~0.09eV in constraining the total neutrino mass for the planned galaxy redshift survey, a factor 2.5 improvement compared to the linear regime. Moreover, this improvement leads to berak the degeneracy between neutrino mass and w. Thus, the refined model prescription offers a vital opportunity to determine both the neutrino masses and dark energy parameter.

Is Dark Energy Composed of Photons with Negative Mass (Dark Photons)?

Dr.  Douglas Snyder (None)

Dark energy may be composed of photons with negative inertial and gravitational mass. Since the photons comprising dark energy would have negative mass, they could not be observed even though they exert a repulsive gravitational force on entities that have positive mass. Furthermore, dark photons possess little mass which is a feature of dark energy. Dark photons are small enough to comprise the needed density to support a universe that is close to flat. They also may allow for repelling entities with positive mass while they are attracted to entities with positive mass, a possible requirement of dark energy.

Type Ia Supernovae Over A Decade in Wavelength

Dr.  Michael Wood-Vasey (Harvard University)

We present preliminary cosmological results from the 6-year ESSENCE supernova survey and discuss our recent work on addressing the current dominant systematic challenge in SN Ia cosmology: distinguishing the nature of dust in supernova host galaxies from the intrinsic color-luminosity variation of SNe Ia. Using data from Swift satellite (UV) and the FLWO 48" (optical) and PAIRITEL (NIR) telescopes at Mt. Hopkins, we are improving our understanding of SNe Ia and beginning to constrain the nature of dust in other galaxies. We find that the NIR H-band is a promising standard-candle reference to determine the impact of intrinsic color and absolute luminosity of SNe Ia and that multiwavelength data is key in separately measuring the dust properties of host galaxies along the line-of-sight to SNe Ia. The next few years should bring significant breakthroughs in this understanding and improvements in our in the use of SNe Ia for determining the nature of dark energy.

Four LSST Probes of Dark Energy

Dr.  Hu Zhan (University of California at Davis)

The half-sky LSST six band survey of four billion galaxies will address dark energy physics by exploiting a diversity of precision probes: Weak lensing tomography (WL) of shear vs. redshift, which probes both distances and the evolution of structure vs. redshift, setting multiple constraints on dark energy. Baryon Acoustic Oscillations (BAO) in galaxy spatial correlations measure distances vs. redshift using the “standard ruler” of the peak in the correlation of dark matter revealed in the temperature anisotropies in the cosmic microwave background (CMB). The redshift distribution of shear peaks due to large structures of dark matter (via WL combined with the optical data) are a potentially sensitive probe of dark energy. Tens of thousands of well-observed supernovae are complementary for probing the recent cosmic era when dark energy becomes dominant. These diverse probes are complementary, removing degeneracies. They form interlocking checks on cosmological models and the physics of dark energy. The joint analysis of BAO and WL is especially powerful, because it utilizes all combinations of galaxy and shear correlations to reduce the uncertainties in the galaxy bias and photo-z error distribution. Astrophysical observations are susceptible to systematics, so the LSST is being specifically designed and engineered to minimize and control systematics at a level ten times below the smallest signal of interest. Systematic error experiments using the Subaru telescope are incorporated in these estimates.